1. Field of the Invention
The present invention relates to a high-frequency transmission line such as a thin flat cable configured to transmit high-frequency signals.
2. Description of the Related Art
To date, for example, thin flat cables for transmitting high-frequency signals in a GHz band have been used in electronic apparatuses. A flat cable is a high-frequency transmission line substantially having, for example, a tri-plate shape, in which ground conductors are arranged on the front surface and back surface of a long dielectric base body and signal lines are arranged within the dielectric base body.
At present, electronic apparatuses perform, for example, GPS communication, wireless LAN communication, and Bluetooth (registered trademark) communication and, hence, handle many high-frequency signals (for example, about 700 MHz or higher). As a result, the flexible substrate (flat cable) disclosed in Japanese Unexamined Patent Application Publication No. 2007-123740 transmits a plurality of high-frequency signals.
The flexible substrate disclosed in Japanese Unexamined Patent Application Publication No. 2007-123740 includes two signal lines separated from each other to suppress crosstalk between the two signal lines.
In some cases, a plurality of high-frequency signals transmitted through a single flat cable have considerably different strengths. For example, the strength of a high-frequency GPS signal is significantly lower than that of a high-frequency signal in wireless LAN communication. Accordingly, in transmission of a signal with such a low strength, it is desirable that the insertion loss of a signal line be made to be sufficiently low.
When the width of a signal line for a GPS signal is increased to reduce the insertion loss of the signal line, the distance between two signal lines is decreased, whereby crosstalk between the two signal lines is generated. On the other hand, when the distance between the two signal lines is maintained constant to suppress the crosstalk, the width of the whole flat cable needs to be increased by an amount corresponding to an increase in the width of the signal line for a GPS signal. Further, when the width of the signal line for a GPS signal is increased, coupling with a ground conductor is likely to be generated, whereby the capacitance of a transmission line formed of the signal line and the ground conductor increases and the characteristic impedance of the transmission line decreases.
Even when the thickness of the signal line is increased instead of increasing the width of the signal line to reduce the insertion loss, the capacitance increases. In other words, the characteristic impedance decreases when the thickness of the signal line is increased. To increase the characteristic impedance, which has decreased, the ground conductor on the front or back surface needs to be spaced apart from the signal line, whereby the thickness of the whole flat cable needs to be increased.